Mandible fractures are common facial injuries, which can occur following severe impacts such as those experienced in motor vehicle accidents and sports. Repair of mandibular fractures first requires bringing the fragments into their correct anatomical position (reduction) and appropriate alignment of the fracture segments so that they can be immobilized (fixated) during fracture healing. When these two goals can be accomplished efficiently and with minimal tissue disruption, the risk of malunion and infection are reduced.
The mandible has an outer surface that is called the outer cortex. Even though this cortical surface is fractured, the bone fragments may not move relative to each other. In general, if the fragments do not move, they are considered stable (or favorable) and are usually managed with conservative techniques, including wiring the upper jaw and the mandible together (intermaxillary fixation or “IMF”) to maintain their pre-existing or normal dental occlusion (the way upper and lower teeth meet). When one of the fragments moves towards the cheek or lips (buccal-labial movement), or towards the tongue (lingual movement), it is unstable (or unfavorable) and surgical methods of open reduction/internal fixation (ORIF) must be considered. Treatment of mandibular fractures using an ORIF technique generally proceeds by first performing IMF and then reducing the fractured bone, and then securing (fixating) the bone in place.
Depending upon the anatomic location and the specific characteristics of the fracture site 100, best seen in FIG. 1, fixation is accomplished through a variety of techniques including drilling holes through the bone and wiring them together (interosseous wiring) (not shown) or by using plates. An internal fixation plate 98 is generally a flat, elongated section of rigid metal containing screw holes at various points along its length for receiving screws to fasten the plate to the bone. One or more plates are placed across a fracture line 100 to fix the bone mass on both sides of the fracture to each other. The plates are secured to the bone with fasteners, usually screws. Interosseous wiring is simple, inexpensive, and needs less exposure of the tissue than that required for plate fixation techniques. It can also reduce and fixate the fracture, but this repair is non-rigid and tends to loosen because of the pressure the thin wire threaded through the bone exerts on the comparatively soft cortical bone. Because the forces the patient exerts when chewing (mastication) exceed the elastic limits of the interosseous wires, this technique not only requires the contemporaneous use of IMF during surgery (as is also done with plating) but also for as long as 6 weeks after surgery. Plate systems, however, can often be used without IMF after surgery.
To facilitate bone fracture healing (osteosynthesis), these fixation systems typically employ metallic hardware, including plate and screws, formed of biocompatible, corrosion resistant metals such as titanium and stainless steel. Systems utilizing resorbable materials have also recently been introduced.
While the main advantage of metallic plates is that they are strong and provide rigid stabilization of the fragments during osteosynthesis, they possess a number of inherent shortcomings. First, in order to accomplish reduction of the fragments, the surgeon must bring the bone fragments into proper alignment. This procedure usually requires the use of a surgical assistant who brings the fragments into alignment and then holds them in position either manually, or with a special tool. Second, because the surface of the mandible is not completely flat, the surgeon typically uses instruments to twist, bend and attempt to conform the conventional flat metal plate to the portion of the mandible onto which it is to be affixed. Shaping and re-shaping the rigid metal plates to conform adequately to bone surfaces is largely accomplished through trial and error. This method, usually conducted while the patient is under anesthesia, increases the requirements for anesthesia and operating room time. If the plate is not shaped correctly to conform to the bone surface, the rigid plate creates an additional problem because during osteosynthesis, bony fragments conform to the plate forcing the bone to heal in an anatomically incorrect position, which may result in dental malocclusion (errors in the way the upper and lower teeth meet to chew food).
Some conventional internal fixation plates have a compression feature that uses the force exerted by tightening the screw in the eccentrically shaped hole through the plate to force the fragments together. When this plate is used, a drill bit is used to drill a hole at the outer edge of the (eccentric compression) hole. A screw is then inserted into the hole and tightened enough to hold the plate in approximate position over the fracture site. The surgeon then turns his attention to the opposite fragment and repeats the procedure by drilling another hole to the outside of the opposite (eccentric compression) hole. The two screws are tightened to obtain compression of the fragments. Two additional screws are then placed through the holes in the outer portion of the plate and the system is stabilized. This technique is not very forgiving. Over or under compression of the fragments can cause displacement. If the mandible is inaccurately positioned, malunion or malocclusion may result. To prevent this undesirable, result, the plate may have to be reapplied in a new position.
Conventional rigid internal fixation plating techniques can also increase the opportunity for complications. For example, the exposure necessary for insertion of large plates can devascularize cortical (outer layer of) bony fragments. Plating on both sides of the mandible (bicortical), also risks injury to the inferior alveolar neurovascular bundle. While rigid plates may effectively restrain the opposing bone fragments against relative movement, as is required to achieve osteosynthesis, when they are not properly positioned, that same rigidity may contribute to bony deformation and malunion.
Rigid fixation of unstable, distracted mandibular fractures is often associated with a “catch-22” problem that requires accurate reduction to fixate while simultaneously needing some method of temporarily fixating the fragments in reduction in order to apply the chosen rigid fixation.
A tension-wire method that uses monocortical screws with stainless-steel wire for fracture reduction and fixation in conjunction with intermaxillary fixation has been described in Wang et al., Arch. Otolaryngol. Head Neck Surg. 124 (April 1998)448-452. In Wang, two screw holes for 2.0-mm-diameters self-tapping titanium or stainless-steel screws, 4 or 6 mm in length, are placed perpendicular to and on each side of the fracture line. Monocortical screws are placed approximately 4 to 6 mm from the fracture line. The screws are then tightened down and then reversed 2 turns to allow a 24-gauge stainless-steel wire loop to be passed around them and fit underneath the head of the first screw. The wire loop, which is tightened around the two screws, both reduces and fixates the opposing sides of the fractured bone. Because the head of the screws are conical, tightening the screws results in further reduction of the fragments.
While the above described tension-wire method (TWM) was originally devised as a method of temporary reduction for rigid fixation, it has been found to be a stable and effective method of fixation. When compared to methods utilizing miniplates, or dynamic compression plates, the TWM also requires less dissection and exposure of the tissues than that required for plating or lag screw techniques, and it is applicable to most simple fractures of the parasymphysis, body, angle, and ramus without the need for external incisions. TWM reduces and fixates the fracture simultaneously and can also be used to reduce an unstable fracture. The TWM is quite strong when two or more planes of fixation can be achieved. Other screw and wire loops can be added to adjust reduction. Despite its advantages, one disadvantage of the TWM alone, which is not encountered with plate and lag screw rigid fixation devices, is the concurrent need for use of IMF, which precludes immediate oral rehabilitation. Because IMF generally supplements the TWM, it should not be used where IMF is contraindicated, such as elderly, debilitated patients and those with increased nutritional demands for whom early oral rehabilitation is important.
Finite element analysis of TWM demonstrates that because the wire is usually plastically deformed while being tightened, it suffers from lack of strength to support biting forces (mastication) during the period of fracture healing. Since IMF must generally supplement TWM, the patient's jaw is required to remain wired for several weeks after the ORIF. While plating devices could be used in conjunction with TWM to dispense with IMF postoperatively, that technique would not solve the problems associated with the use of rigid metal plates.
The TWM is comparatively quick and easy to use because it simultaneously combines reduction and fixation, does not increase the complication rate, and has a low cost. This makes TWM an attractive alternative to current methods of mandibular internal reduction and fixation for simple and/or unstable fractures. In order to meet all the goals of mandibular fracture repair however, and reduce the problems encountered with existing internal fixation techniques using metal plates, utilize the benefits of TWM, and eliminate the need for IMF after surgery, a new device is desirable.